1. Field of the Invention
The present invention relates generally to the field of telecommunications. More particularly, the present invention relates to an improved RF amplitude filter.
2. Description of the Related Art
Amplitude filters are used to protect sensitive receiving amplifiers, such as, for example, an LNA or Low Noise Amplifier which are particularly useful for phase controlled antennas. The receiving amplifiers in transmitter-receiver modules, what are known as T/R modules must be protected from high power signals. The transmission and the reception frequency of such systems lie in the radio frequency X band, for example. Average powers of one watt and peak powers of 10 watts are made available by a transmitting amplifier (HPA, High Power Amplifier). Consequently, high power reception signals can enter the input of the receiving amplifier.
There are three different modes of operation for the amplitude filters which are: 1) the pass mode (Low Insertion Loss Receive Mode) which is used for the reception of weak signals; 2) the filtration mode (High Insertion Loss Receive Mode) that is used in the reception of signals with high power; and 3) the block mode or Isolation Mode that may be selected from outside through a control signal and which serves to isolate the receiving amplifier from the antenna.
It is absolutely crucial that an absorbent amplitude filter absorb the received signal while in the filtration mode and block mode. The signal must not be reflected back to the input of the circuit.
There are two possibilities for realization of such an absorbent filter. The basis for both cases is what is known as a reflective amplitude filter consisting of a .lambda./4 line and preferably two PIN diodes. The two PIN diodes are respectively connected at the input and output sides opposite a reference potential.
In the first design, before the input of the reflective filter, a switch is connected at whose first input the antenna is connected, and at whose second input the receiving amplifier is connected through the filter. The third terminal of the switch is connected to a load resistor. This load resistor absorbs the reflected signal.
In the second design, the received signal is split with a first power splitter into preferably two equal parts and merged again at the output with a second power splitter. Two reflective amplitude filters are located in-line between the two power splitters. In the pass mode, the two subsignals pass through the filter undisturbed. In the filtration mode and the block mode, the two subsignals of the reception signal are reflected with a phase difference of 180 degrees. No signal power is therefore reflected back to the input of the circuit.
It is an object of the invention to provide a filter which is both physically small and which requires few electrical components. Other objects and advantages of the present invention will be apparent from the following Summary and Detailed Description of the Preferred Embodiments.